This invention relates to lowering bit error rates in superconductor integrated circuit devices, and, more particularly, to a superconductor means for producing useful double flux quantum pulses in response to received single flux quantum pulses.
Metals, metal alloys and ceramics found to exhibit zero electrical resistance are commonly referred to as superconductors. Typically, those superconductors don""t attain the superconductive state unless cooled to extremely low temperatures, referred to as cryogenic temperatures. Each such superconductor material possesses a unique cryogenic temperature, referred to as the transition temperature (xe2x80x9cTcxe2x80x9d), at which the respective metal, metal alloy, or ceramic becomes superconducting, changing in electrical resistance from a measurable or relatively high value of resistance to zero. One known superconductor is niobium, a refractory metal, which transitions to a superconducting state at a temperature of 9.2 Kelvin.
Superconductor digital electronic devices have previously been constructed of superconductors and the functionality of such devices demonstrated. As example, with a zero-resistance characteristic during superconductivity, electrical current induced into a loop formed of the superconductor metal persists indefinitely. With appropriate drivers and sensors, the foregoing loop may serve as a digital memory. When the direction of the current induced in the loop is in a clockwise direction the memory state may represent a xe2x80x9c1xe2x80x9d digital bit; when the direction of induced current is counterclockwise, the memory state may represent the bit xe2x80x9c0xe2x80x9d.
Superconductor digital electronics devices have been fabricated as integrated circuits on a silicon wafer using the photo-lithographic mask and etch techniques or other known techniques most familiar to those in the semiconductor industry. Such superconductor integrated circuit devices provide the desired functionality in a very small package or chip. Superconductor devices operate at very high speeds, as example, 100 GHz to 770 GHz, and very low power, which is unattainable with present semiconductor devices. Because of the high speeds of operation and low power requirement, superconductor electronic devices remain attractive for many applications.
A principal element to the construction of a superconductor digital electronic device is the Josephson junction. The Josephson junction is formed, as example, of two layers of superconductors, such as niobium, separated by a very thin layer of electrical insulation, such as aluminum oxide. When cooled to the transition temperature and biased with DC current below a certain critical current, (xe2x80x9cICxe2x80x9d) the Josephson junction is superconducting and the junction conducts current without developing a voltage drop there across and without dissipation of energy, exhibiting no electrical resistance. Consequently, the junction does not produce heat, which is a significant advantage for integrated circuits. If biased above the critical current, the Josephson junction produces an RF signal, consisting of a series of pulses at RF frequencies. Thus, the critical current is a boundary at which the electrical properties of the junction changes as described.
Superconductor circuits utilize the foregoing property of the junction to regenerate single flux quantum (xe2x80x9cSFQxe2x80x9d) pulses. The time integral of the voltage of a single flux quantum pulse is physical constant approximately equal to 2.07 millivolt picoseconds or, in alternate terms, 2.07 milliamp picohenrys (e.g., h/2e, where h is Plank""s constant and e is an electron charge). When a SFQ pulse is applied to a Josephson junction that is properly DC biased below the respective critical current, the current produced by the SFQ pulse when added to the DC bias current may cause the Josephson junction to brief exceed the critical current. The Josephson junction then undergoes a 360 degree shift in quantum phase or, as otherwise termed, electronically xe2x80x9cflips-over.xe2x80x9d In undergoing that shift the Josephson junction reproduces the single flux quantum pulse in response a the applied SFQ pulse.
In superconducting integrated circuit (xe2x80x9cICxe2x80x9d) devices containing multiple Josephson junctions, the junctions are formed on a common superconductor metal layer, referred to as a ground plane, deposited over an insulator substrate, such as silicon. The multiple Josephson junction devices may be logically divided into groups of two or more junctions, the groups, sometimes referred to as xe2x80x9cSQUIDsxe2x80x9d (an acronym for superconducting quantum interference device). For example, a single flux quantum pulse transmission line, referred to as a Josephson transmission line, may be formed of a number of SQUIDs arranged in serial order, each SQUID containing two Josephson junctions connected electrically in parallel in a superconducting loop, the latter also sometimes referred to as a Josephson loop.
A single flux quantum pulse applied to the input of the Josephson transmission line (xe2x80x9cJTLxe2x80x9d), may be said to propagate along the transmission line to the output, moving from SQUID to SQUID in that line, and thence to the electrical load connected to the output of the transmission line. In fact, the SFQ pulse is regenerated at each Josephson junction (stage), which can produce current and power gain. The transmission line may in total contain two or more Josephson junctions, the number of Josephson junctions (and SQUIDs) that form the transmission line can be increased to traverse the desired distance.
Digital integrated circuits require superconductor chip-to-chip communication to convey Single Flux Quanta. The assignee of the present invention has internally demonstrated chip-to-chip communication of SFQ pulses at rates of up to 20 Gbps on a superconductor chip mounted on a passive superconductor carrier. However, the bit error rate (xe2x80x9cBERxe2x80x9d) was found to increase with frequency and exceeded a self-imposed limit of 10exe2x88x9216 at frequencies over five Gbps. The presence of attenuation and reflections (e.g. transients) on the chip-to-chip transmission line and other electronic noise which mask or obscure the digital bits at random is a principal factor causing bit errors. Those influences increase in adverse effect as the frequencies increase to 20 Gpbs, to 40 Gpbs, and more so at 100 Gpbs, the more desirable frequency regions for communication, and produce unacceptable bit error rates. The need to remove those influences, or to minimize the adverse affect of those influences or to otherwise lower the BER is apparent if superconductor chip-to-chip communication at those frequencies is ever to succeed.
When the foregoing kind of problem occurs in non-superconductor electronic devices commonly used at lower frequencies, such as semiconductor circuits, a known solution is to amplify the desired signal, that is, employ a driver to amplify and apply the amplified signal to the succeeding xe2x80x9cnoisyxe2x80x9d stages of the communications equipment, thereby raising the level of signal relative to the electronic noise, the signal-to-noise ratio. By definition, a driver is a device that supplies a useful amount of signal energy to another device to insure the proper operation of the latter device. By upgrading the signal relative to the noise, the succeeding stages in the electronic apparatus more readily recognizes the signal, and, hence, the error rate is minimized or eliminated.
Although a Josephson junction is an active device, the junction cannot function as a signal amplifier in the customary sense. The only superconductor devices heretofore known in the superconductor art that are capable of obtaining operational speeds of twenty to one hundred Gbps with a negligible BER generate single flux quantum pulses. Otherwise, no superconductor drivers have been known previously. The unavailability of a useful superconductor driver would be expected to lead one to explore other alternatives for lowering the bit error rate. However, as an advantage, the present invention provides a new and useful superconductor driver.
An object of the present invention is to reduce the bit error rate in operation of superconductor digital communications circuits, and, in particular, in superconductor chip-to-chip communication.
Another object of the invention is to provide a driver for superconductor circuits.
Still another object of the invention is to enable effective superconductor chip-to-chip communications at frequencies of 20, 40 and 100 Gpbs.
A further object of the invention is to increase the strength of the data pulses in a superconductor device; to effectively multiply a single flux quantum pulse in size.
And a still further object of the invention is to provide a superconductor driver capable of providing a flux quantum pulse greater in size than a single flux quantum pulse and use that larger pulse to drive subsequent superconductor circuits.
In accordance with the foregoing objects and advantages, a superconductor driver is formed by combining a rapid SFQ one-way buffer together in series with a Josephson transmission line that is lightly loaded. It is found that in response to each SFQ pulse applied to the input of the one-way buffer, a double flux quantum pulse is generated at the transmission line output. Preferably, to ensure light loading the load on the transmission line is at least three to eight times greater in resistance than the shunt resistance necessary to critically damp the Josephson Junction.
SFQ pulses applied to the input of the driver propagate through the one-way buffer and along the Josephson transmission line. Because the Josephson transmission line emulates an open-circuit line, the SFQ pulse triggers the final Josephson junction in the transmission line to generate a flux quantum pulse voltage that is twice as great as that of the incident SFQ pulse. That double flux quantum pulse may be usefully applied to light loads. The one-way buffer blocks SFQ pulses reflected from the output end of the transmission line from reaching the input, thereby protecting the input.
In accordance with an additional aspect to the invention, the JTL is under-damped. A more specific aspect to the invention is that a Josephson junction serves as the one-way gate to the buffer. Joining two known superconductor devices, the Josephson one-way buffer and the lightly loaded Josephson transmission line, together into a combination, new, unanticipated and useful results are achieved.
The foregoing and additional objects and advantages of the invention, together with the structure characteristic thereof, that were only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.